Method for rejuvenating refractory articles

ABSTRACT

Refractory metal products damaged by oxidation are restored to their original condition by heating the damaged products in a reducing atmosphere to a temperature adequate to successively reduce the oxide to the metal and sinter the metal.

United States Patent Inventor Creuie E. l-lolcomhe, Jr.

Oak Ridge, Tenn.

Appl. No. 844,202

Filed July 23, 1969 Patented Nov. 23, 1971 Assignee The United States of America as represented by the United States Atomic Energy Commission METHOD FOR REJUVENATING REFRACTORY ARTICLES [56] References Cited UNITED STATES PATENTS 1,318,452 10/1919 Keyes 75/224 1,377,982 5/1921 Reyes... 75/224 1,663,547 3/1928 Gero 75/224 2,1 10,893 3/1938 Sendzimir. 1 17/51 2,122,053 6/1938 Burkhardt 75/224 2,185,410 1/1940 Lederer 75/224 2,227,177 12/1940 Berghaus et a1, 75/224 2,240,821 5/1941 Young 75/224 Primary Examiner- Reuben Epstein Attorney-Roland A. Anderson ABSTRACT: Refractory metal products damaged by oxidation are restored to their original condition by heating the damaged products in a reducing atmosphere to a temperature adequate to successively reduce the oxide to the metal and sinter the metal.

METHOD FOR RIHUVENATING REFRACTORY ARTICLES BACKGROUND OF THE INVENTION The present invention relates generally to the rejuvenation of refractory metals, and more particularly to the rejuvenation of oxidation-damaged refractory metal products by reducing the oxide to metal and sintering the metal. This invention was made in the course of, or under, a contract with the U. S. Atomic Energy Commission.

Refractory metals, because of their stability at high temperatures and resistance to corrosion by various liquid media, are frequently fabricated into products or structures for use in environments subject to high temperature and/or corrosive conditions. However, refractory metals suffer a shortcoming or drawback in that, when subjected to elevated temperatures in the presence of oxygen, these metals oxidize. Repeated or prolonged exposures to oxidizing conditions cause refractory metal structures to undergo a considerable and often deleterious loss of strength such as breakdown of the walls of the structure due to scaling and eventual sagging and cracking. It has been common practice to replace a refractory metal structure damaged by oxidation with a newly fabricated structure. This practice is somewhat undesirable since the cost of refrac tory metals is high and the method of manufacturing such metals into suitable structures is time consuming. For example, a simple cylindrical tantalum crucible weighing about l ,400 grams costs about three hundred and fifty dollars for the tantalum and requires about l is hours fabrication time.

SUMMARY OF THE INVENTION Refractory metal structures or products damaged by oxidation are rejuvenated without destroying the structure. This rejuvenation or restoration is accomplished by confining the oxidation-damaged structure in a furnace; contacting the structure with a reducing atmosphere; thereafter, prior to, or concurrently with this contacting, heating the confined structure to a temperature adequate to successively reduce the oxide to metal and sinter together the reduced metal particulates; and maintaining the confined structure at temperature for a duration sufficient to reduce and sinter virtually all the oxided metal.

By practicing the present invention the heretofore expensive replacement of oxidation-damaged refractory metal structures is obviated, thereby making the use of refractory metal structures more economically feasible and desirable.

DETAILED DESCRIPTION In order to provide a more facile understanding of the present invention, the following description is directed to the rejuvenation or restoration of oxidation-damaged tantalum structures. However, it is to be understood that, without changing the oxide reducing and metal sintering conditions, other oxidation-damaged refractory metals such as molybdenum, tungsten, and niobium may be similarly rejuvenated.

The carbon monoxide reductant as employed in the description below in connection with the rejuvenation of tantalum structures may be replaced by reducing gases more suitable for the particular refractory oxide being reduced; for example, a hydrogen atmosphere may be employed when reducing molybdenum and tungsten. Other reductants useable include gaseous hydrocarbons such as methane, propane, ethane, and the like.

The 1,600 C. temperature employed for the reduction and sintering operations has proven to be satisfactory for the tantalum as well as for niobium. Lower reduction temperatures must be used for molybdenum and tungsten oxides. The reduction temperature depends upon the particular metal being reduced and varies in a range from about 900 to l,800 C., with care being exercised to insure that the temperature remains below the melting point of the oxide being treated but at the same time adequate to sinter the particular refractory metal being treated. For example, when reducing and sintering tantalum and niobium, the temperature is preferably within a range of about l,400 to 1,800" O, whereas, when reducing molybdenum and tungsten, the temperature is preferably within a range of about 900 to l ,400" C.

A tantalum crucible employed in a noncarbon furnace in an argon atmosphere becomes oxidized after repeated heatings at temperatures greater than about 300 C. if sufficient oxygen is present in the furnace atmosphere. This oxidation causes the formation of scales of oxided metal on the crucible walls, which weakens the walls and tends to cause them to sag and crack.

The oxidation-damaged tantalum crucible is readily rejuvenated by practicing the following method. The damaged crucible is placed in a carbon furnace and provided with an argon atmosphere. A trace amount of oxygen is then leaked into the argon atmosphere of the furnace to combine with the carbon of the furnace and form an atmosphere of carbon monoxide for reducing the tantalum oxide to metallic tantalum when the furnace and i1; contents are heated to a temperature of about l,600 C. The reduction of the tantalum oxide to metal is apparently provided by the following reaction:

Essentially simultaneously with the reduction of the tantalum oxide to metal, sintering of the metal particles resulting from the reduction occurs to ensure that the initial configuration of the crucible is retained. While it is not entirely clear why the reduced metal sinters at such a relatively low temperature, it is believed that the presence of the reducing atmosphere and the substantially pure reduced metal makes the sintering possible. During the reduction-sintering operation there is no evidence of fusion of the tantalum since such would have a somewhat glassy appearance. Also, no volatilization of the oxides occurs since the melting point of tantalum oxide is well about 1,800 C. After heating the damaged tantalum crucible for a period of about 4 hours, in the reducing atmosphere of the furnace the reduction and sintering operation is adequately completed. The furnace is then cooled and the restored crucible removed and reused or stored for subsequent use.

It will be seen that the present invention provides a unique method of rejuvenating refractory metals damaged by oxidation. The rejuvenated refractory metals have all the desirable properties and characteristics restored therein that were present when the structure was initially fabricated. Also, the restoration of oxidation-damaged refractory products may be repeated several times without suffering any undesirable drawbacks.

What is claimed is:

I. In the art of using a refractory metal product in an oxygen containing environment at elevated temperatures wherein the refractory metal is from the group consisting of tantalum, niobium, molybdenum and tungsten, and or prolonged exposure to said environment to detract from strength required of the refractory metal product for use in said environment, a method of restoring the refractory metal forming said product to virtually its metallic condition prior to said oxidation comprising the steps of confining the refractory product in a reducing atmosphere, heating the confined product to a temperature less than melting point of the refractory metal oxide but sufficient to successively and essentially simultaneously effect a reaction between the reducing atmosphere and the refractory metal oxide to reduce the latter to the refractory metal and to sinter the reduced refractory metal, and maintaining the product at said temperature for a duration adequate to reduce virtually all the metal oxide and sinter virtually all the metal oxide reduced to the refractory metal.

2. The method as claimed in claim I, wherein the refractory metal is tantalum, the reducing atmosphere is selected from the group of reductants consisting essentially of carbon monoxide and gaseous hydrocarbons, and wherein said temperature is selected from a range of 1,400 to l ,800' C.

3. The method as claimed in claim I, wherein the refractory metal is molybdenum or tungsten, the reducing atmosphere is selected from the group of reductants consisting essentially of hydrogen, carbon monoxide, and gaseous hydrocarbons, and

4. The method as claimed in claim I, wherein the refractory metal is niobium, the reducing atmosphere is selected from the group of reductants consisting essentially of carbon monoxide and gaseous hydrocarbons, and wherein said temwherein said temperature is selected from a range of 900 C. 5 peratu'e is selected from a range of 1,400, to

to l,400 C.

It i i I i g UNITIED S'IA'IES PATENT OFFICE CERTIFICATE OF. CORRECTION Patent No. 3,622 312 Dated November 23, 197] Inventor(s) Cressie E. Holcombe. Jr,

It is certified that error appears in the above-identified patent and that aid Letters Patent are hereby corrected as shown below:

Column 2, line 25 shou1d read 5C0 Ta 0 2Ta 5C0 Iine 36, "about" shou1d read ---above---; line 55, a ter "and", second occurrence, read ---where1n sufficient oxidation of the refractory metal occurs due to repeated---.

Signed and sealed this Lpch day of July 1972.

(SEAL) Attest:

mama]: ILFLETCHERJR. ROBERT GOTTSCHALK Attesting Offi cer- Commissioner of Patents 

2. The method as claimed in claim 1, wherein the refractory metal is tantalum, the reducing atmosphere is selected from the group of reductants consisting essentially of carbon monoxide and gaseous hydrocarbons, and wherein said temperature is selected from a range of 1,400* to 1,800* C.
 3. The method as claimed in claim 1, wherein the refractory metal is molybdenum or tungsten, the reducing atmosphere is selected from the group of reductants consisting essentially of hydrogen, carbon monoxide, and gaseous hydrocarbons, and wherein said temperature is selected from a range of 900* C. to 1,400* C.
 4. The method as claimed in claim 1, wherein the refractory metal is niobium, the reducing atmosphere is selected from the group of reductants consisting essentially of carbon monoxide and gaseous hydrocarbons, and wherein said temperature is selected from a range of 1,400* C. to 1,800* C. 